dt-bindings: clock: rcar-gen2: Remove obsolete header files

The clock definitions in <dt-bindings/clock/r8a779?-clock.h> were
superseded by those in <dt-bindings/clock/r8a779?-cpg-mssr.h> a long
time ago.

The last DTS user of these files was removed in commit 362b334b
("ARM: dts: r8a7791: Convert to new CPG/MSSR bindings") in v4.15.
Driver support for the old bindings was removed in commit
58256143 ("clk: renesas: Remove R-Car Gen2 legacy DT clock
support") in v5.5, so there is no point to keep on carrying these.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Acked-by: Conor Dooley <conor.dooley@microchip.com>
Link: https://lore.kernel.org/d4abb688d666be35e99577a25b16958cbb4c3c98.1718796005.git.geert+renesas@glider.be